Kitabı oku: «The Evolutionist at Large», sayfa 5

XI.
AMONG THE HEATHER

This is the worst year for butterflies that I can remember. Entomologists all over England are in despair at the total failure of the insect crop, and have taken to botanising, angling, and other bad habits, in default of means for pursuing their natural avocation as beetle-stickers. Last year's heavy rains killed all the mothers as they emerged from the chrysalis; and so only a few stray eggs have survived till this summer, when the butterflies they produce will all be needed to keep up next season's supply. Nevertheless, I have climbed the highest down in this part of the country to-day, and come out for an airing among the heather, in the vague hope that I may be lucky enough to catch a glimpse of one or two old lepidopterous favourites. I am not a butterfly-hunter myself. I have not the heart to drive pins through the pretty creatures' downy bodies, or to stifle them with reeking chemicals; though I recognise the necessity for a hardened class who will perform that useful office on behalf of science and society, just as I recognise the necessity for slaughtermen and knackers. But I prefer personally to lie on the ground at my ease and learn as much about the insect nature as I can discover from simple inspection of the living subject as it flits airily from bunch to bunch of bright-coloured flowers.

I suppose even that apocryphal person, the general reader, would be insulted at being told at this hour of the day that all bright-coloured flowers are fertilised by the visits of insects, whose attentions they are specially designed to solicit. Everybody has heard over and over again that roses, orchids, and columbines have acquired their honey to allure the friendly bee, their gaudy petals to advertise the honey, and their divers shapes to ensure the proper fertilisation by the correct type of insect. But everybody does not know how specifically certain blossoms have laid themselves out for a particular species of fly, beetle, or tiny moth. Here on the higher downs, for instance, most flowers are exceptionally large and brilliant; while all Alpine climbers must have noticed that the most gorgeous masses of bloom in Switzerland occur just below the snow-line. The reason is, that such blossoms must be fertilised by butterflies alone. Bees, their great rivals in honey-sucking, frequent only the lower meadows and slopes, where flowers are many and small: they seldom venture far from the hive or the nest among the high peaks and chilly nooks where we find those great patches of blue gentian or purple anemone, which hang like monstrous breadths of tapestry upon the mountain sides. This heather here, now fully opening in the warmer sun of the southern counties – it is still but in the bud among the Scotch hills, I doubt not – specially lays itself out for the bumblebee, and its masses form about his highest pasture-grounds; but the butterflies – insect vagrants that they are – have no fixed home, and they therefore stray far above the level at which bee-blossoms altogether cease to grow. Now, the butterfly differs greatly from the bee in his mode of honey-hunting; he does not bustle about in a business-like manner from one buttercup or dead-nettle to its nearest fellow; but he flits joyously, like a sauntering straggler that he is, from a great patch of colour here to another great patch at a distance, whose gleam happens to strike his roving eye by its size and brilliancy. Hence, as that indefatigable observer, Dr. Hermann Müller, has noticed, all Alpine or hill-top flowers have very large and conspicuous blossoms, generally grouped together in big clusters so as to catch a passing glance of the butterfly's eye. As soon as the insect spies such a cluster, the colour seems to act as a stimulant to his broad wings, just as the candle-light does to those of his cousin the moth. Off he sails at once, as if by automatic action, towards the distant patch, and there both robs the plant of its honey and at the same time carries to it on his legs and head fertilising pollen from the last of its congeners which he favoured with a call. For of course both bees and butterflies stick on the whole to a single species at a time; or else the flowers would only get uselessly hybridised instead of being impregnated with pollen from other plants of their own kind. For this purpose it is that most plants lay themselves out to secure the attention of only two or three varieties among their insect allies, while they make their nectaries either too deep or too shallow for the convenience of all other kinds. Nature, though eager for cross-fertilisation, abhors 'miscegenation' with all the bitterness of an American politician.

Insects, however, differ much from one another in their æsthetic tastes, and flowers are adapted accordingly to the varying fancies of the different kinds. Here, for example, is a spray of common white galium, which attracts and is fertilised by small flies, who generally frequent white blossoms. But here, again, not far off, I find a luxuriant mass of the yellow species, known by the quaint name of 'lady's bedstraw' – a legacy from the old legend which represents it as having formed Our Lady's bed in the manger at Bethlehem. Now why has this kind of galium yellow flowers, while its near kinsman yonder has them snowy white? The reason is that lady's bedstraw is fertilised by small beetles; and beetles are known to be one among the most colour-loving races of insects. You may often find one of their number, the lovely bronze and golden-mailed rose-chafer, buried deeply in the very centre of a red garden rose, and reeling about when touched as if drunk with pollen and honey. Almost all the flowers which beetles frequent are consequently brightly decked in scarlet or yellow. On the other hand, the whole family of the umbellates, those tall plants with level bunches of tiny blossoms, like the fool's parsley, have all but universally white petals; and Müller, the most statistical of naturalists, took the trouble to count the number of insects which paid them a visit. He found that only 14 per cent. were bees, while the remainder consisted mainly of miscellaneous small flies and other arthropodous riff-raff; whereas in the brilliant class of composites, including the asters, sunflowers, daisies, dandelions, and thistles, nearly 75 per cent. of the visitors were steady, industrious bees. Certain dingy blossoms which lay themselves out to attract wasps are obviously adapted, as Müller quaintly remarks, 'to a less æsthetically cultivated circle of visitors.' But the most brilliant among all insect-fertilised flowers are those which specially affect the society of butterflies; and they are only surpassed in this respect throughout all nature by the still larger and more magnificent tropical species which owe their fertilisation to humming-birds and brush-tongued lories.

Is it not a curious, yet a comprehensible circumstance, that the tastes which thus show themselves in the development, by natural selection, of lovely flowers, should also show themselves in the marked preference for beautiful mates? Poised on yonder sprig of harebell stands a little purple-winged butterfly, one of the most exquisite among our British kinds. That little butterfly owes its own rich and delicately shaded tints to the long selective action of a million generations among its ancestors. So we find throughout that the most beautifully coloured birds and insects are always those which have had most to do with the production of bright-coloured fruits and flowers. The butterflies and rose-beetles are the most gorgeous among insects: the humming-birds and parrots are the most gorgeous among birds. Nay more, exactly like effects have been produced in two hemispheres on different tribes by the same causes. The plain brown swifts of the North have developed among tropical West Indian and South American orchids the metallic gorgets and crimson crests of the humming-bird: while a totally unlike group of Asiatic birds have developed among the rich flora of India and the Malay Archipelago the exactly similar plumage of the exquisite sun-birds. Just as bees depend upon flowers, and flowers upon bees, so the colour-sense of animals has created the bright petals of blossoms; and the bright petals have reacted upon the tastes of the animals themselves, and through their tastes upon their own appearance.

XII.
SPECKLED TROUT

It is a piece of the common vanity of anglers to suppose that they know something about speckled trout. A fox might almost as well pretend that he was intimately acquainted with the domestic habits of poultry, or an Iroquois describe the customs of the Algonquins from observations made upon the specimens who had come under his scalping-knife. I will allow that anglers are well versed in the necessity for fishing up-stream rather than in the opposite direction; and I grant that they have attained an empirical knowledge of the æsthetic preferences of trout in the matter of blue duns and red palmers; but that as a body they are familiar with the speckled trout at home I deny. If you wish to learn all about the race in its own life you must abjure rod and line, and creep quietly to the side of the pools in an unfished brooklet, like this on whose bank I am now seated; and then, if you have taken care not to let your shadow fall upon the water, you may sit and watch the live fish themselves for an hour together, as they bask lazily in the sunlight, or rise now and then at cloudy moments with a sudden dart at a May-fly who is trying in vain to lay her eggs unmolested on the surface of the stream. The trout in my little beck are fortunately too small even for poachers to care for tickling them: so I am able entirely to preserve them as objects for philosophical contemplation, without any danger of their being scared away from their accustomed haunts by intrusive anglers.

Trout always have a recognised home of their own, inhabited by a pretty fixed number of individuals. But if you catch the two sole denizens of a particular scour, you will find another pair installed in their place to-morrow. Young fry seem always ready to fill up the vacancies caused by the involuntary retirement of their elders. Their size depends almost entirely upon the quantity of food they can get; for an adult fish may weigh anything at any time of his life, and there is no limit to the dimensions they may theoretically attain. Mr. Herbert Spencer, who is an angler as well as a philosopher, well observes that where the trout are many they are generally small; and where they are large they are generally few. In the mill-stream down the valley they measure only six inches, though you may fill a basket easily enough on a cloudy day; but in the canal reservoir, where there are only half-a-dozen fish altogether, a magnificent eight-pounder has been taken more than once. In this way we can understand the origin of the great lake trout, which weigh sometimes forty pounds. They are common trout which have taken to living in broader waters, where large food is far more abundant, but where shoals of small fish would starve. The peculiarities thus impressed upon them have been handed down to their descendants, till at length they have become sufficiently marked to justify us in regarding them as a separate species. But it is difficult to say what makes a species in animals so very variable as fish. There are, in fact, no less than twelve kinds of trout wholly peculiar to the British Islands, and some of these are found in very restricted areas. Thus, the Loch Stennis trout inhabits only the tarns of Orkney; the Galway sea trout lives nowhere but along the west coast of Ireland; the gillaroo never strays out of the Irish loughs; the Killin charr is confined to a single sheet of water in Mayo; and other species belong exclusively to the Llanberis lakes, to Lough Melvin, or to a few mountain pools of Wales and Scotland. So great is the variety that may be produced by small changes of food and habitat. Even the salmon himself is only a river trout who has acquired the habit of going down to the sea, where he gets immensely increased quantities of food (for all the trout kind are almost omnivorous), and grows big in proportion. But he still retains many marks of his early existence as a river fish. In the first place, every salmon is hatched from the egg in fresh water, and grows up a mere trout. The young parr, as the salmon is called in this stage of its growth, is actually (as far as physiology goes) a mature fish, and is capable of producing milt, or male spawn, which long caused it to be looked upon as a separate species. It really represents, however, the early form of the salmon, before he took to his annual excursion to the sea. The ancestral fish, only a hundredth fraction in weight of his huge descendant, must have somehow acquired the habit of going seaward – possibly from a drying up of his native stream in seasons of drought. In the sea, he found himself suddenly supplied with an unwonted store of food, and grew, like all his kind under similar circumstances, to an extraordinary size. Thus he attains, as it were, to a second and final maturity. But salmon cannot lay their eggs in the sea; or at least, if they did, the young parr would starve for want of their proper food, or else be choked by the salt water, to which the old fish have acclimatised themselves. Accordingly, with the return of the spawning season there comes back an instinctive desire to seek once more the native fresh water. So the salmon return up stream to spawn, and the young are hatched in the kind of surroundings which best suit their tender gills. This instinctive longing for the old home may probably have arisen during an intermediate stage, when the developing species still haunted only the brackish water near the river mouths; and as those fish alone which returned to the head waters could preserve their race, it would soon grow hardened into a habit engrained in the nervous system, like the migration of birds or the clustering of swarming bees around their queen. In like manner the Jamaican land-crabs, which themselves live on the mountain-tops, come down every year to lay their eggs in the Caribbean; because, like all other crabs, they pass their first larval stage as swimming tadpoles, and afterwards take instinctively to the mountains, as the salmon takes to the sea. Such a habit could only have arisen by one generation after another venturing further and further inland, while always returning at the proper season to the native element for the deposition of the eggs.

These trout here, however, differ from the salmon in one important particular beside their relative size, and that is that they are beautifully speckled in their mature form, instead of being merely silvery like the larger species. The origin of the pretty speckles is probably to be found in the constant selection by the fish of the most beautiful among their number as mates. Just as singing birds are in their fullest and clearest song at the nesting period, and just as many brilliant species only possess their gorgeous plumage while they are going through their courtship, and lose the decoration after the young brood is hatched, so the trout are most brightly coloured at spawning time, and become lank and dingy after the eggs have been safely deposited. The parent fish ascend to the head-waters of their native river during the autumn season to spawn, and then, their glory dimmed, they return down-stream to the deep pools, where they pass the winter sulkily, as if ashamed to show themselves in their dull and dusky suits. But when spring comes round once more, and flies again become abundant, the trout begin to move up-stream afresh, and soon fatten out to their customary size and brilliant colours. It might seem at first sight that creatures so humble as these little fish could hardly have sufficiently developed aesthetic tastes to prefer one mate above another on the score of beauty. But we must remember that every species is very sensitive to small points of detail in its own kind, and that the choice would only be exerted between mates generally very like one another, so that extremely minute differences must necessarily turn the scale in favour of one particular suitor rather than his rivals. Anglers know that trout are attracted by bright colours, that they can distinguish the different flies upon which they feed, and that artificial flies must accordingly be made at least into a rough semblance of the original insects. Some scientific fishermen even insist that it is no use offering them a brown drake at the time of year or the hour of day when they are naturally expecting a red spinner. Of course their sight is by no means so perfect as our own, but it probably includes a fair idea of form, and an acute perception of colour, while there is every reason to believe that all the trout family have a decided love of metallic glitter, such as that of silver or of the salmon's scales. Mr. Darwin has shown that the little stickleback goes through an elaborate courtship, and I have myself watched trout which seemed to me as obviously love-making as any pair of turtle-doves I ever saw. In their early life salmon fry and young trout are almost quite indistinguishable, being both marked with blue patches (known as 'finger-marks') on their sides, which are remnants of the ancestral colouring once common to the whole race. But as they grow up, their later-acquired tastes begin to produce a divergence, due originally to this selective preference of certain beautiful mates; and the adult salmon clothes himself from head to tail in sheeny silver, while the full-grown trout decks his sides with the beautiful speckles which have earned him his popular name. Countless generations of slight differences, selected from time to time by the strongest and handsomest fish, have sufficed at length to bring about these conspicuous variations from the primitive type, which the young of both races still preserve.

XIII.
DODDER AND BROOMRAPE

This afternoon, strolling through the under-cliff, I have come across two quaint and rather uncommon flowers among the straggling brushwood. One of them is growing like a creeper around the branches of this overblown gorse-bush. It is the lesser dodder, a pretty clustering mass of tiny pale pink convolvulus blossoms. The stem consists of a long red thread, twining round and round the gorse, and bursting out here and there into thick bundles of beautiful bell-shaped flowers. But where are the leaves? You may trace the red threads through their labyrinthine windings up and down the supporting gorse-branches all in vain: there is not a leaf to be seen. As a matter of fact, the dodder has none. It is one of the most thorough-going parasites in all nature. Ordinary green-leaved plants live by making starches for themselves out of the carbonic acid in the air, under the influence of sunlight; but the dodder simply fastens itself on to another plant, sends down rootlets or suckers into its veins, and drinks up sap stored with ready-made starches or other foodstuffs, originally destined by its host for the supply of its own growing leaves, branches, and blossoms. It lives upon the gorse just as parasitically as the little green aphides live upon our rose-bushes. The material which it uses up in pushing forth its long thread-like stem and clustered bells is so much dead loss to the unfortunate plant on which it has fixed itself.

Old-fashioned books tell us that the mistletoe is a perfect parasite, while the dodder is an imperfect one; and I believe almost all botanists will still repeat the foolish saying to the present day. But it really shows considerable haziness as to what a true parasite is. The mistletoe is a plant which has taken, it is true, to growing upon other trees. Its very viscid berries are useful for attaching the seeds to the trunk of the oak or the apple; and there it roots itself into the body of its host. But it soon produces real green leaves of its own, which contain the ordinary chlorophyll found in other leaves, and help it to manufacture starch, under the influence of sunlight, on its own account. It is not, therefore, a complete drag upon the tree which it infests; for though it takes sap and mineral food from the host, it supplies itself with carbon, which is after all the important thing for plant-life. Dodder, however, is a parasite pure and simple. Its seeds fall originally upon the ground, and there root themselves at first like those of any other plant. But, as it grows, its long twining stem begins to curl for support round some other and stouter stalk. If it stopped there, and then produced leaves of its own, like the honeysuckle and the clematis, there would be no great harm done: and the dodder would be but another climbing plant the more in our flora. However, it soon insidiously repays the support given it by sending down little bud-like suckers, through which it draws up nourishment from the gorse or clover on which it lives. Thus it has no need to develop leaves of its own; and it accordingly employs all its stolen material in sending forth matted thread-like stems and bunch after bunch of bright flowers. As these increase and multiply, they at last succeed in drawing away all the nutriment from the supporting plant, which finally dies under the constant drain, just as a horse might die under the attacks of a host of leeches. But this matters little to the dodder, which has had time to be visited and fertilised by insects, and to set and ripen its numerous seeds. One species, the greater dodder, is thus parasitic upon hops and nettles; a second kind twines round flax; and the third, which I have here under my eyes, mainly confines its dangerous attentions to gorse, clover, and thyme. All of them are, of course, deadly enemies to the plants they infest.

How the dodder acquired this curious mode of life it is not difficult to see. By descent it is a bind-weed, or wild convolvulus, and its blossoms are in the main miniature convolvulus blossoms still. Now, all bind-weeds, as everybody knows, are climbing plants, which twine themselves round stouter stems for mere physical support This is in itself a half-parasitic habit, because it enables the plant to dispense with the trouble of making a thick and solid stem for its own use. But just suppose that any bind-weed, instead of merely twining, were to put forth here and there little tendrils, something like those of the ivy, which managed somehow to grow into the bark of the host, and so naturally graft themselves to its tissues. In that case the plant would derive nutriment from the stouter stem with no expense to itself, and it might naturally be expected to grow strong and healthy, and hand down its peculiarities to its descendants. As the leaves would thus be rendered needless, they would first become very much reduced in size, and would finally disappear altogether, according to the universal custom of unnecessary organs. So we should get at length a leafless plant, with numerous flowers and seeds, just like the dodder. Parasites, in fact, whether animal or vegetable, always end by becoming mere reproductive sacs, mechanisms for the simple elaboration of eggs or seeds. This is just what has happened to the dodder before me.

The other queer plant here is a broomrape. It consists of a tall, somewhat faded-looking stem, upright instead of climbing, and covered with brown or purplish scales in the place of leaves. Its flowers resemble the scales in colour, and the dead-nettle in shape. It is, in fact, a parasitic dead-nettle, a trifle less degenerate as yet than the dodder. This broomrape has acquired somewhat the same habits as the other plant, only that it fixes itself on the roots of clover or broom, from which it sucks nutriment by its own root, as the dodder does by its stem-suckers. Of course it still retains in most particulars its original characteristics as a dead-nettle; it grows with their upright stem and their curiously shaped flowers, so specially adapted for fertilisation by insect visitors. But it has naturally lost its leaves, for which it has no further use, and it possesses no chlorophyll, as the mistletoe does. Yet it has not probably been parasitic for as long a time as the dodder, since it still retains a dwindling trace of its leaves in the shape of dry purply scales, something like those of young asparagus shoots. These leaves are now, in all likelihood, actually undergoing a gradual atrophy, and we may fairly expect that in the course of a few thousand years they will disappear altogether. At present, however, they remain very conspicuous by their colour, which is not green, owing to the absence of chlorophyll, but is due to the same pigment as that of the blossoms. This generally happens with parasites, or with that other curious sort of plants known as saprophytes, which live upon decaying living matter in the mould of forests. As they need no green leaves, but have often inherited leafy structures of some sort, in a more or less degenerate condition, from their self-supporting ancestors, they usually display most beautiful colours in their stems and scales, and several of them rank amongst our handsomest hot-house plants. Even the dodder has red stalks. Their only work in life being to elaborate the materials stolen from their host into the brilliant pigments used in the petals for attracting insect fertilisers, they pour this same dye into the stems and scales, which thus render them still more conspicuous to the insects' eyes. Moreover, as they use their whole material in producing flowers, many of these are very large and handsome; one huge Sumatran species has a blossom which measures three feet across. On the other hand, their seeds are usually small and very numerous. Thousands of seeds must fall on unsuitable places, spring up, and waste all their tiny store of nourishment, find no host at hand on which to fasten themselves, and so die down for want of food. It is only by producing a few thousand young plants for every one destined ultimately to survive that dodders and broomrapes manage to preserve their types at all.